CONWAY - As Kieran Ramsey looked around the law enforcement command center at the police department Thursday, he saw lots of familiar faces.

Ramsey, an FBI agent assigned to the Boston field office who is leading the bureau's part of the search for Abigail Hernandez, saw Senior Associate Attorney General Jane Young, many senior state police and New Hampshire Fish and Game officers whom he has worked with on child-abduction cases. They are part of a team of about 100 law enforcement agents, including 35 or so FBI agents, trying to find Abigail, 15, last seen Oct. 9.

So far, searches of more than 3,000 acres and 5 square miles of town haven't turned up a trace of Abigail or her missing iPhone, which last transmitted a data update at 3:07 p.m. Oct. 9, he said.

There is no evidence to suggest she was abducted or ran away, Ramsey said.

There is no evidence indicating anything about her disappearance, he said, but investigators won't stop looking.

"What makes it especially tough is many of us in this room are parents, too, and we know time is of the essence," Ramsey said.

The size and scope of this investigation is not unusual for cases when minors are involved, Ramsey said. A missing person, defined in this context as someone who has lost contact with family and friends, will generally bring a large response from state and federal agencies in New Hampshire because many police departments don't have the manpower, expertise and equipment necessary to handle such cases.

Local law enforcement keeps the FBI apprised of all missing-children cases, Ramsey said. The case of a Nashua 13-year-old who left home and returned last week is an example. Ramsey said there was adequate information that the boy was not in danger and was a runaway.

The difference in the case of a runaway is "the risk for them (having been) abducted is eliminated."

The FBI has more than 200 officers in northern New England that "slide" to various cases at any one time.

"You would see this kind of response in any case like this," Young said. "The FBI and the other agencies bring assets to the table that are needed.''

Many missing-person cases, especially those involving adults, can be handled by local police, but a larger presence is needed when a child is involved, said Senior Assistant Attorney General Jeffery Strelzin.

The size of the presence is determined by the situation. "The facts dictate the response," he said.

"The younger a missing child is, the less likely it is that they could take care of themselves," Strelzin said. "If you have a 14- or 15-year-old child, she is not old enough to have a driver's license, she has no financial means to take care of herself, it's very concerning, and time is of the essence, so that will draw a big response."

Bill Barry, a retired 17-year veteran tracker and missing-child investigator with the Hillsborough County Sheriff's Department, said investigators are poring over large amounts of information - online and off - to find clues.

"Particularly if you don't have a lot of information on her whereabouts, you need a lot of people there, as many resources as possible," he said.

"There's a lot to do, they have to go back over weeks and months of everything she did, and see what was going on in her life, who was she friends with, did she make any new friends, did anybody see her with somebody new. It's very involved, and it takes time."

Strelzin said missing-person cases can take weeks of thorough investigation by law enforcement officers. After that, if a missing person is not found, the case will remain open for a period, sometimes years, with authorities monitoring developments.

"The active part stops when you run out of avenues of information to pursue," he said.

Barry says Abigail's family and friends should remain hopeful. He investigated six missing children in his career, and all six were found, alive and well, within weeks or months of their disappearance.

"You can't give up on these cases," he said. "I'm hoping for her. There's a good chance she could be found."